The telephone services of the related art are based on voice communication using telephones, so when using videophones or other supplementary services, special terminals (dedicated hardware or dedicated software) separate from the telephones that have been used so far have had to be prepared. At that time, when the user wished to use a supplementary service after communication with a telephone had begun, the user had to hang up the telephone and then redial with the special terminal.
Accordingly, technology has been proposed which stores in advance media that can be accessed by a user and can be handled by a plurality of terminals, manages the plurality of terminals as a single virtual terminal, determines the media corresponding to an event upon occurrence of the event, and selects the terminal corresponding to this media from among the plurality of terminals (refer to Patent Document 1).
An overview of an example of operation is described below. FIG. 1 is a view explaining the operation of Patent Document 1 in brief.
In a communication system 1, terminals usable by user A or B are controlled virtually as single terminals. When the user A calls user B by a videophone call from a videophone terminal 2, the user B cannot utilize a videophone terminal, but can utilize voice and video separately at the terminals 4 and 5, so video communication is begun between the videophone terminal 2 of the user A and the voice terminal 4+supplementary service terminal 5 of the user B.    Patent Document: Japanese Laid-Open Patent Publication No. 2006-270837
However, the above-mentioned technology had the following issues. FIG. 2 is a view for explaining the issues of the related art.
(1) The user has no way of knowing what supplementary service communication media (video terminal etc.) is available with the other user before the connection of the supplementary service.
(2) A registered predetermined communication terminal is selected according to a priority rank held in advance in the communication system 1, so the user is unable to dynamically select a communication terminal according to the other user or the usage situation.
(3) When the speech communication session by the voice terminal ends, the user also has to separately carry out end processing for the currently established supplementary service communication media.
For example, there is no way of knowing what supplementary service communication media the other user supports, so even if in an environment where file sharing is possible, there is no way of knowing if file sharing is possible until actual connection of the supplementary service is set up.
Further, a terminal is automatically selected according to a priority rank stored in advance in a terminal control unit of the communication system 1, so, for example, even when the user A can access communication media for the same supplementary service (video) by a plurality of terminals (video phone terminal) 2 and (supplementary service terminal) 3 and also when the user A wishes to receive the video only for a certain call on a wide-screen terminal 3 (low priority), the user A will only be able to select the videophone terminal 2 of high priority preferentially.